1. Field of the Invention
Embodiments of the present invention generally relate to a method of manufacturing semiconductor devices. More specifically, embodiments of the present invention relate to methods of removing high-concentration dopants near the surface of a substrate.
2. Description of the Related Art
In semiconductor manufacturing processes, use of doping agents is often desired. Doping refers to any of a number of processes for implanting impurities into an otherwise substantially pure material. The impurities are desired for some property they impart to the doped composition or some change they effect in the properties of the composition. In some applications, dopants may provide an interface barrier to prevent diffusion of two materials into each other at an interface. For example, a surface of a dielectric material interfacing with a conductive layer in a film capacitor may be doped to prevent diffusion of conductive elements into the dielectric layer. In other applications, dopants may change thermal properties of a material. For example, dopants may be implanted in a material to be heat treated in order to change the thermal or optical properties of the material to facilitate the treatment. In a common application, dopants are distributed throughout a region of a substrate to form source and drain junctions for a transistor device. For example, dopants may be infused into a silicon crystal. The dopants have a different electronic configuration from the silicon, creating the potential for current flow through the crystal.
The process of infusing dopants into a material generally follows one of two paths. Dopants may be deposited on the surface of the material to be doped and then “driven-in” by heating the material to encourage the dopants to diffuse into the material. This process will usually involve forming a thermally conductive but robust capping layer over the dopant layer to prevent sublimation of the deposited dopant during heat treatment. Dopants will diffuse into the substrate material during heat treatment, resulting in a concentration gradient generally higher near the surface of the material and lower further into the material. The longer and more intense the heat treatment, the more the diffusion, and the flatter the concentration gradient. In processes involving implantation of dopants into a silicon crystal, this thermal treatment process also serves to “activate” the dopant atoms by encouraging them to occupy positions in the crystal lattice, and it increases order generally through the crystal lattice, reducing electrical resistivity due to crystal dislocations.
An alternate path involves energetic implantation of dopant ions into a substrate. In this process, dopants are ionized into a plasma, either remotely or in situ, and an electromagnetic field is used to accelerate the ions toward the substrate. The ions strike the surface of the substrate and burrow into the crystal structure. The depth each ion burrows into the crystal depends mostly on the kinetic energy of the ion. As in the “drive-in” embodiment above, the concentration distribution generally decreases monotonically with depth, and annealing is similarly done to diffuse and activate the dopants.
In each process, the region to be implanted with dopant may be “amorphized” prior to or during implantation. Amorphizing the region disrupts the crystal structure of the substrate, creating conduits for dopant atoms or ions to infiltrate the substrate. Amorphizing generally results in deeper implantation because dopants encounter fewer collisions near the surface than when implanted without amorphizing. This can be advantageous when deep implantation is desired.
Both processes result in the highest concentration of dopant remaining near the surface of the substrate. The former process frequently results in significant quantities of dopant being left on the surface of the substrate. In either case, after annealing, the substrate may be removed from the process chamber and placed into a storage box for a period of time. During that time, the substrate frequently degasses. In particular, the highly concentrated dopants near the surface of the substrate react with moisture in the air to form volatile compounds. Some of these are also highly toxic. For example, arsenic (As) and phosphorus (P), two widely used dopants, react with moisture in air to form arsine (AsH3) and phosphine (PH3), both of which are highly toxic. OSHA allowable exposure of arsine, for example, has recently been lowered from 50 parts-per-billion (ppb) to 5 ppb, due to its toxicity. Therefore, a method is needed for removing high concentrations of dopants from regions near the surface of a doped substrate.